Switching power supply apparatuses are used as alternating current (AC)/direct current (DC) converters or DC/DC converters. Conventionally, there are isolated switching power supply apparatuses, each having a primary circuit and a secondary circuit. In an AC/DC converter, the primary circuit receives electric power from an AC power supply, and the secondary circuit outputs a DC voltage. The primary circuit and the secondary circuit are electrically isolated from each other, and magnetically coupled with each other by a transformer. Thus, in the isolated switching power supply apparatuses, when an electrical short circuit occurs in one of the primary and the secondary circuits, the influence of the short circuit is prevented from being exerted on the other. For example, even when an overvoltage occurs in the primary circuit when the primary circuit is struck by lightning, any device connected with the secondary circuit is protected.
In addition, there are synchronous rectifier circuits, each of which rectifies a voltage waveform produced across the secondary winding of a transformer. Specifically, each of the synchronous rectifier circuits uses a transistor (hereinafter referred to also as a switch) connected to the secondary winding, and turns on or off the switch at a timing determined in accordance with the voltage waveform. In a case where a rectifier circuit is constituted by a diode and a capacitor, the on-voltage of the diode and the current which flows through the diode will cause significant power loss, and thus limit the conversion efficiency of the converter, which includes the rectifier circuit. In the synchronous rectifier circuits, however, since the transistor has a low on-voltage, the power loss is reduced.
In recent years, dedicated control integrated circuits (ICs) are often used to accurately control the switch and improve the conversion efficiency. For example, the control ICs each operate with a power supply voltage, which is obtained by rectifying a voltage waveform across the secondary winding by using a diode, a resistor; and capacitor, and generates a control voltage (pulse wave) which is to be supplied to the switch. The maximum value of the control voltage is produced by an internal-voltage generation circuit of a corresponding control IC. Some of the control ICs have an external terminal which is connected to a capacitor. The capacitor is charged with the voltage generated by the internal-voltage generation circuit, to ensure a sufficient amount of current to drive the switch.
By the way, some new transistors have recently been developed, having a lower on-resistance and a lower switching loss, compared to the power silicon (Si)—metal-oxide-semiconductor field-effect transistor (MOSFET). The new transistors are, for example, FETs having a wide bandgap semiconductor. Examples of the FETs include a gallium nitride (GaN)—high electron mobility transistor (HEMT), and a MOSFET having silicon carbide (SiC). If such a transistor is used as the switch of the switching power supply apparatus, the conversion efficiency would be increased.
See, for example, Japanese Laid-open patent Publication Nos. 2010-130708, 2008-245387, and 2017-79527.
However, since the maximum value of the control voltage, which is supplied to the switch by a conventional control IC, is often set for the Si-MOSFET, the maximum value may exceed a withstand voltage of the gate terminal of the above-described transistor, which has a low on-resistance resistance and a low switching loss. For this reason, when the transistor (hereinafter referred to also as a low withstand-voltage-of-gate transistor) is used as the switch, some components, including a dedicated driver used to lower the control voltage, will be added. This disadvantageously increases the number of components.